Recrystallization | MIT Digital Lab Techniques Manual

Summary

This video demonstrates the recrystallization technique used for purifying crystalline solids, specifically focusing on naphthalene. The process involves selecting an appropriate solvent, dissolving the sample, removing impurities through hot filtration and decoloration, cooling to induce crystallization, and finally collecting and drying the purified crystals. The video explains both single and two-solvent recrystallization methods, emphasizing careful solvent selection and proper filtration techniques. It provides detailed steps for each stage of the procedure, helping viewers understand both theory and practice to become skilled experimentalists in the lab.

Takeaways

• 😀 Recrystallization is a key technique in chemistry for purifying crystalline solids, especially for chemists working in the lab.
• 😀 The process involves five main steps: choosing an appropriate solvent, dissolving the sample, decolorizing, cooling to induce crystallization, and collecting the purified crystals.
• 😀 Selecting the right solvent is critical; the ideal solvent dissolves the compound at high temperatures but not at room temperature.
• 😀 Solubility tests using a hot water bath help determine the best solvent for recrystallization by testing how well the compound dissolves in different solvents.
• 😀 Activated charcoal is used to remove colored impurities from the solution, but caution must be taken to avoid absorbing the compound with the charcoal.
• 😀 A hot filtration step helps remove insoluble contaminants by using a fluted filter paper, ensuring the solution stays hot to prevent premature crystallization.
• 😀 After the hot filtration, the solution is boiled to remove excess solvent, then cooled to promote slow crystal formation.
• 😀 If crystals do not form upon cooling, techniques like scratching the flask or adding a seed crystal can encourage crystallization.
• 😀 Once crystallization begins, placing the flask in an ice bath helps complete the process, ensuring maximum crystal yield.
• 😀 The final step involves collecting the purified crystals through vacuum filtration and washing them with cold solvent to remove residual impurities.
• 😀 For compounds that require a two-solvent recrystallization, choosing compatible solvents that are miscible and have the desired solubility properties is essential for success.

Q & A

What is the purpose of recrystallization in chemistry?

Recrystallization is a purification technique used to purify crystalline solids by removing impurities.

What are the main steps involved in the recrystallization process?

The main steps include choosing an appropriate solvent, dissolving the sample in minimal solvent, decolorizing and performing hot filtration, cooling the solution to induce crystallization, and collecting, washing, and drying the purified crystals.

How do you determine the appropriate solvent for recrystallization?

The appropriate solvent should dissolve the sample at its boiling point but not at room temperature. Solubility tests are performed using different solvents to identify the best choice.

What is the importance of using a minimal amount of solvent when dissolving the sample?

Using a minimal amount of solvent ensures better recovery of crystals, as it increases the concentration of the compound in the solution, which aids in efficient crystallization.

What role does activated charcoal play in recrystallization?

Activated charcoal is used to remove colored impurities from the solution. It absorbs the impurities, which can then be removed by filtration.

Why should you avoid heating organic solvents over an open flame?

Organic solvents are flammable, and heating them over an open flame increases the risk of fire and explosion.

What should be done if crystallization does not occur after cooling to room temperature?

If crystallization does not occur, you can try scratching the flask or adding a seed crystal to initiate the crystallization process.

Why is it important to allow the recrystallization solution to cool slowly?

Allowing the solution to cool slowly promotes the growth of large, pure crystals by providing time for the molecules to arrange into an orderly structure.

What is the purpose of an ice bath in the recrystallization process?

The ice bath is used to complete the crystallization process by lowering the temperature, which drives the formation of more crystals.

How do you perform a vacuum filtration to collect the purified crystals?

To perform a vacuum filtration, use a cold solvent to rinse the crystals and pull air through them using a vacuum. This helps to remove excess solvent and dry the crystals more efficiently.